Carbon fibers (hereinafter simply referred to as “CF”) in general have excellent properties such as high strength, high elastic modulus, high electrical conductivity and high heat conductivity, and are being widely used in various composite materials by taking advantage of these properties. The uses thereof are not limited to products in the field of using the mechanical properties of CF, such as high strength and high elastic modulus, but also include applications as a filler. For example, CF can be used as a filler for releasing heat from electronic instruments such as personal computers and portable telephones by using the high heat conductivity of CF or carbon materials to overcome the heat generation trouble ascribable to miniaturization, high densification, high performance and the like of these electronic devices or parts. By using their high electrical conductivity, CF can be used as an electrically conducting resin filler for electromagnetic wave shielding materials, antistatic materials and the like or as a filler for use in the electrostatic coating for resin accompanying a reduction in weight of motor vehicles. Furthermore, by using the properties as a carbon material, such as chemical stability, heat stability and fine structure, CF could be used as a field emission material such as in a flat display.
CF is conventionally produced as a so-called organic carbon fiber which is produced by heat-treating and thereby carbonizing fiber such as PAN-(polyacrylonitrile), pitch- or cellulose-based fiber. In the case of using these CF fibers as a filler for fiber reinforced composite materials, the contact area with the base material is increased to elevate the reinforcing effect, for example, by reducing the fiber diameter or increasing the fiber length.
However, the reduction in diameter of the organic fiber as a starting material of CF is limited and can give at best a fiber diameter of from 5 to 10 μm depending on the production method and fine CF having a fiber diameter of 1 μm or less, particularly on the order of 10 to 200 nm, cannot be produced. Even if such fine CF could be produced, the ratio of length to fiber (i.e., aspect ratio) is limited and production with good profitability cannot be attained. As such, CF having a small diameter and a large aspect ratio has been not industrially produced, though this has been demanded.
On the other hand, studies of vapor grown carbon fiber started in the latter half of 1980s. The production method of these fibers is utterly different from those of organic fibers. As for vapor grown carbon fiber (hereinafter simply referred to as “pyrolytic CF”), it is known that pyrolytic CF having a diameter of 1 μm or less, approximately tens of nm, can be obtained by the vapor phase pyrolysis of a gas such as hydrocarbon in the presence of a metal catalyst.
For example, a method of introducing an organic compound as a starting material, such as benzene, into a high-temperature reaction furnace together with an organic transition metal compound catalyst such as ferrocene using a carrier gas and producing pyrolytic CF on a substrate (see, JP-A-60-27700 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)), a method of producing pyrolytic CF in a floated state (see, JP-A-60-54998), or a method of growing pyrolytic CF on the wall of a reaction furnace (see, Japanese Patent No. 2778434) have been proposed. The pyrolytic CF produced by these methods is heat-treated at a high temperature to remove any attached pyrolysis products and enhance the crystallinity and the obtained final vapor grown carbon fiber (this carbon fiber is hereinafter simply referred to as “VGCF”; pyrolytic CF, VGCF and the like are sometimes collectively called “vapor grown carbon fiber”) is used for various uses.
By these production methods, vapor grown carbon fiber having a high electrical conductivity, excellent heat conductivity, a fine diameter size and a large aspect ratio and being suitable for filler materials can be obtained. At present, VGCF having a diameter of approximately from 10 to 200 nm and an aspect ratio of approximately from 10 to 500 is easily mass-produced and used as an electrically or thermally conducting filler material, for example, in fillers of electrically conducting resin or in additives of a lead storage battery.
This vapor grown carbon fiber is characterized by the shape or crystal structure and the fiber has a structure such that carbon hexagonal network crystals are stacked by rolling annular rings into a cylindrical shape and an extremely thin hollow part is present in the center part.
The crystallinity of vapor grown carbon fiber is easily enhanced by the graphitization as compared with conventional PAN-based CF and pitch-based CF, however, due to reduction in the fiber diameter to the order of 10 to 200 nm, graphite crystal is difficult to grow and poor crystallinity results as compared with natural graphite.
In order to enhance the crystallinity of VGCF, there has been previously disclosed a method of adding a boron compound during the graphitization of pyrolytic CF and thereby doping boron into VGCF, so that the growth of graphite crystal can be promoted and VGCF improved in the crystallinity can be obtained.
For improving the heat-releasing property of electronic devices, an electrical insulating material having high heat conductivity is necessary as a heat-releasing filler and for this purpose, alumina and the like are predominantly used. Particularly, accompanying the miniaturization and high densification of electronic devices in recent years, a heat-releasing filler having higher heat conductivity is demanded and to satisfy this requirement, alumina is formed into a spherical shape to improve the filling density in a composite material and thereby elevate the heat conductivity, or aluminum nitride and the like having higher heat conductivity are being used as a heat-releasing filler. The heat-releasing member of an electric device must be electrically insulating in many cases and the heat-releasing filler is also demanded to be an electrically insulating and highly heat-conductive heat-releasing filler.
At present, in the case where the electrical insulating property is necessary, fine particles of an inorganic material such as alumina and aluminum nitride are used as the heat-releasing filler, though the heat conductivity is slightly unsatisfactory. The heat release is attained by the heat conduction passing through the contact points between fine particles of the filler and thus, the contact point is rate-determining for the amount of heat released and stands as an obstacle to the heat conduction. On the other hand, since the filler is a fine particle, the contact area for the heat conduction is small and the number of contact points where the heat conduction passes through is large. Therefore, the heat releasing property is greatly reduced. Accordingly, for improving the heat release, it is preferred to reduce the number of contact points, for example, to use a thin fibrous heat-releasing filler.